1. Field of the Invention
The present invention relates to a method of converting a negative image into a positive image, which is needed in order to execute various types of image processing based on a negative film, e.g., when a negative image of a negative film is read by a scanner and a printed matter printing a positive image is to be created. The present invention relates also to a record medium for recording a program which is used to execute such an image conversion method on a computer.
2. Description of the Background Art
A positive film allows to recognize the contents of the film by directly looking at the film. Further, after obtaining image data through color separation in which a positive film is separated into three colors of red (hereinafter "R"), green (hereinafter "G") and blue (hereinafter "B") by image inputting equipment such as a scanner, it is easy to reproduce and display an image which corresponds to the image data on a monitor or the like.
On the other hand, a negative film does not allow to grasp the contents of the film unless a negative image of an object which is photographed on the film is somehow reversed into a positive image. While it is considered that a quality of a positive image which is obtained by an image inversion method of reversing a negative image into a positive image is largely affected by whether the image inversion method is appropriate or not, there is no document available which discloses a specific image inversion method of inverting an image through electric image processing.
An image inversion method as above may be as follows. For example, after separating a negative film through color separation into three colors of R, G and B with a scanner and reading image data of a negative image, intensity conversion for reversing brightness and darkness is executed by electrical signal processing , whereby a highlight point and a shadow point for each color in the negative image as it is before inversion are correlated with a highlight point and a shadow point for each color in the positive image as it is after inversion. As herein termed, with respect to a negative image, a "highlight point" refers to a point which is to be expressed as the brightest point in a positive image which is obtained by inversion, and a "shadow point" refers to a point which is to be expressed as the darkest point in the positive image which is obtained by inversion.
Describing the intensity conversion mentioned above more precisely, intensity conversion formulas each in the form of a direct function is established for converting minimum values for the respective colors of image data which are obtained from a negative image into maximum values for the respective colors of image data of a positive image and for converting maximum values for the respective colors of the image data which are obtained from the negative image into minimum values for the respective colors of the image data of the positive image. Data conversion is executed which applies the intensity conversion formulas to data of each pixel in the negative image, thereby obtaining a positive image which is inversion of the negative image.
However , when combination of highlight points for the respective colors of a negative image as it is before inverted is a relatively bright chromatic color, the method as described above does not permit to evenly invert the chromatic color, and therefore, it is not possible to obtain a positive image which resembles an object.
Further, the method as described above does not necessarily guarantee that a bright white portion in an object, for example, becomes a highlight point in a negative image. The bright white portion in the object may be colored, in which case it is not possible to obtain a positive image which resembles the object.
This problem is considered to be because of a characteristic called an inter-image effect which is ascribed by a manufacturer to a negative film for the purpose of brightly reproducing R, G and B.
This characteristic produces an effect that color development densities of R, G and B increase when a film is exposed with separated light of R, G and B rather than when the film is exposed with white light. That is, when an object contains a white portion and a highly color saturated portion, but for the inter-image effect, a density of the white portion becomes the highest with respect to R, G and B. The inter-image effect, however, increases a B-density in a highly color saturated portion of a blue color higher than a B-density in the white portion, for example. The expression a "high (dark) density" is herein used in the context of a negative image, and therefore, in the case of a positive image, refers to a bright portion. As described above, a B-value of RGB-values on a negative image of a blue color plane, for instance, becomes much smaller than a B-value of RGB-values on the negative image of a white color plane in some cases, which in turn gives slight R- and G-tints to the white color plane.